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Sunday, 18 March 2012

Noise

In this section we are examining digital noise. Digital noise is often compared to grain in film but in fact the two are not the same. Digital noise is a sampling 'error' which can and often does result in the degradation of part of the image. Film grain is a necessary part of the chemical process that is developing and printing film.

Influencing factors on digital noise are the length of an exposure or a high ISO setting.

Prior to this exercise I have looked at the images 'Grey Texture' and 'Turkish Dance' in the Key Resources section of the student web site. This goes to show that distinguishing noise from a real pattern can be difficult as well as subjective.

To carry out this exercise I have set my camera to aperture priority and carried out a little test to make sure my slowest shutter speed is no longer than 1/2 a second. I settled on an aperture of F8. The camera was mounted on a tripod and I set up a house-plant with some white foam board as the background. I made sure the white foam board was in the shadow.

I then started shooting the same setup from my lowest aperture which is 50 ISO to the highest which is 25600. The ISO range I went through is as follows: 50 (L), 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 5000, 6400, 12800 (marked as H1) and finally 25600 (marked as H2). L, H1 and H2 are classed as ISO expansion in the camera manual (Canon EOS 5D MKII) and are not normally available unless a custom function is set.

Here is the 50 ISO image which will be the least noisy.

50 ISO F8. The lowest noise image

To make the details easier to see I have zoomed in to 100% on an area that shows both background and the foreground subject.

Detail from the 50 ISO image above.

We can assume a steady increase in noise as the ISO gets higher. Just to make it easier to see, lets compare the same detail in this shot to the highest ISO picture which is H1 (25600 ISO is ISO expansion. The normal highest ISO would be 6400).

Same detail as above but at 25600 ISO.

When comparing the two in this manner it is easy to see the degradation in the second image. The plain back-ground is now suffering from a lot of blocks of pink/purple and blue/green and no longer appears as a smooth, plain colour. The green stem is showing the pink/purple blocks more than the green with the latter presumably being swallowed up by the green of the stem. The pink of the flower is being disrupted and is showing mainly blue noise.

This noise is at its most extreme when using ISO expansion. When you look at the same section at 6400 which is the camera's normal top ISO setting, things are considerably better. The coloured noise is still evident in the background but the flower and the stem are a lot cleaner with the noise not so apparent. The main difference between this photo and the first 50 ISO version is that in this image the detail does not come across as being quite so smooth.

The petal at ISO 6400, the normal top ISO for the Canon EOS 5D MKII.

The final image I have added is at ISO 800. Things are again a lot smoother than the previous shot and I would say this is perfectly usable under normal circumstances. Compare the edges of the green stem and the smoothness of the background to the 6400 ISO version. Note also that the petal does not seem to be that different.

ISO 800. Perfectly usable under normal conditions.

Examining the rest of the pictures shows a gradual increase in quality as we move nearer the 50 ISO. Once below 800 ISO the differences become so subtle that it is sometimes difficult to spot the reduction in noise on a picture by picture basis.

I think that the noise may have shown up more prominently had I had a darker shadow area judging by the effects of noise on the dark stem of the plant.